Multiple types of power supplies exist that can be used to supply a small amount of supply power to an individual integrated circuit. One type of integrated circuit that sometimes needs to be powered is a microcontroller integrated circuit. A typical microcontroller integrated circuit might require only about ten milliamperes of supply current at a low supply voltage of 3.3 volts. A way to provide this small amount of power to the microcontroller integrated circuit at a low cost without wasting a lot of power is desired.
FIG. 1 (Prior Art) is a diagram of a first type power supply circuit 1 usable for this purpose. Power supply circuit 1 is called a linear regulator. A bridge rectifier 2 converts an AC input voltage from a source 15 into a rectified signal VR on node 3. If, for example, the AC input voltage is 120 VAC 60 Hz ordinary wall power, then the peak of the rectified signal VR across capacitor 4 may be about +169 volts. If, for example, the AC input voltage is 240 VAC 50 Hz wall power, then the peak of the rectified signal VR across capacitor 4 is about +338 volts. If only ten milliamperes at 3.3 volts is needed to supply power to the microcontroller 5, then the linear regulator is quite inefficient. Considering the 240 VAC situation, the voltage drop across the linear regulator 6 is approximately 338 volts minus 3.3 volts. At an average current flow of ten milliamperes, approximately 3.3 watts of power is dissipated by linear regulator 6. The power needed is only 33 mW (10 mA multiplied by 3.3 volts). Efficiency of the power supply is therefore about 33 mW/3.3 W, or 0.1 percent efficiency.
FIG. 2 (Prior Art) is a simplified diagram of a second type of power supply circuit 8 usable for the purpose of supplying power to microcontroller 5. The power supply circuit 8 is a flyback converter. The flyback converter 8 is about 80 to 85 percent efficient when in normal steady state operation, but the flyback converter 8 has several problems. First, the flyback converter 8 has relatively expensive and large components such as transformer 9 and output voltage sense circuitry 10-14. Second, the control circuitry 16 and the output voltage sense circuitry 10-14 of the flyback converter 8 must be powered. During initial start up of the flyback converter 8, before the auxiliary winding 17 can supply power to the control circuitry, power is supplied to the control circuitry 16 via resistor 18. Supplying power from the high rectified voltage VR on node 19 through this resistor 18 to a lower supply voltage used to power the control circuitry 16 is inefficient. Efficiency of the switching power supply during initial startup is similar to that of the linear regulator described above.
In some applications, the overall circuit being powered is powered up, and powered down, frequently. When the circuit is in a powered down condition, the flyback converter is not powered and is not switching. Next, the overall circuit is to power up and operate for a short period in a normal operating mode. The switching power supply therefore is powered up, and operates in an initial start up mode until it is operating in regulation, at which point the switching power supply starts operating in its normal operating mode. After only a brief amount of time in normal operating mode operation, the overall circuit is powered down again. The switching power supply is therefore powered down along with the other parts of the overall circuit. Because the period of normal mode operation is so short as compared to the amount of time the switching power supply spends in its initial start up mode, the efficiency of the switching power supply is undesirably low.